The present invention relates to a method of initializing recording medium and, more particularly, to a method of initializing phase change recording medium for high density recording.
A material which can reversibly assume a crystal state and an amorphous (or non-crystal) state, is used for a recording film of the phase change recording medium. The phase change recording medium to be initialized is an optical recording medium, which has such a structure as to detect the difference between its reflectances in the crystal and amorphous states as a signal.
Materials usually used for phase change recording are those of chalcogenite type, e.g., GsSbTe type, InSbTe type, InSe type, InTe type, AsTeGe type, TeOx-GeSn type, TeSeSn type, SbSeBi type, BiSeGe type, AgInSbTe type, etc. These materials are used to form a film on a substrate of acrylic acid resin, polycarbonate resin, etc., thus obtaining a recording medium. Right after the manufacture of the recording medium, however, the recording material used for forming the film is entirely in the amorphous state.
The recording film in the amorphous state right after its manufacture is unstable, and also its reflectance is low. In this state of the recording film, it is difficult to obtain focus servo and track servo. To solve this problem, the recording film should be entirely initialized by carrying out an initializing process on the recording medium.
In the usual initializing method, the recording medium is irradiated with a laser beam to elevate the temperature of the phase change recording medium up to the melting temperature thereof, and then the film is crystallized by cooling for a certain period of time. This initializing process is very important in view of determining the subsequent characteristics of the phase change recording film. This is so because the characteristics of the recording film greatly vary in dependence on the way of initialization.
Heretofore, the phase change recording film is initialized by irradiating it with a laser beam at a certain fixed power level. It is well known in the art that the crystal state of the phase change recording film that is obtained in this method, is slightly different from the crystal state obtained in case of carrying out recording and erasing on an actual recording medium reproducing apparatus. This is so because the phase change recording film obtained after the actual recording and erasing is a result of repeated heating and cooling.
FIG. 4 is a view showing a relationship between the power level of the laser beam used in the prior art recording medium initialization method and time. As is seen from the Figure, the laser beam power level is fixed. The laser beam irradiation is made by using an apparatus as shown in FIG. 5. The same apparatus is also used for data reproduction. As shown, in the apparatus a recording medium which is in the form of a disc is initialized continuously by causing movement of the laser beam from its inner periphery toward its outer periphery or vice versa while causing its rotation.
As shown above, where the phase change recording medium is different in the crystal state right after its initialization and after carrying out the recording and erasing several times, its optical characteristics are also different in these different states. That is, in this case the recording medium is different in the optical characteristics right after the initialization and after carrying out over-writing (i.e., recording and erasing) several tens of times.
Some attempts for providing improvement in this connection have been proposed. For example, Japanese Patent Laid-Open Publication No. 4-366424 shows a method, in which a phase change recording film is irradiated with a laser beam of a certain fixed power level to make portions of the film irradiated with the laser beam spot to be in a crystal state. The position of the laser beam spot on the recording medium is gradually shifted. In this way, the recording film is initialized. This initialization is carried out such that fused crystals having relatively large crystal grain sizes are left.
The fused crystals are in a crystal state close to the state obtainable by recording and erasing data with the actual recording medium reproducing apparatus, thus solving the above problem. In this method, however, only part of the laser beam spot is used for the initialization of the phase change recording film. Therefore, the method has a drawback that it takes a very long time to initialize the entire recording medium.
Japanese Patent Laid-Open Publication No. 6-12670 shows a different initializing method, which uses an irradiating laser beam with the power level thereof in a pulse form. In this prior art initializing method, however, the initialization can be made only track by track, and therefore requires a great deal of time. Besides, this method requires formation of a reference signal in the phase change recording film of the recording medium. In other words, the method requires a preamble requirement, and is therefore very limitative.
Japanese Patent Laid-Open Publication No. 4-102228 proposes a further initializing method, which again uses an irradiating laser beam with the power level thereof in a pulse form. This method has an aim of facilitating the focus servo and track servo as described below. As described above, while a recording medium right after the initialization is usually entirely in a crystal state, the reflection intensity in this state is different from the mean reflection intensity in case when signal is recorded with an actual recording medium reproducing apparatus.
As described before, this phenomenon results in some load on the focus servo and track servo. In the proposed initializing method, record marks which are equivalent to recording signal, are preliminarily written on the recording medium to eliminate the difference of the reflection intensity from the mean reflection intensity in the case when signal is recorded with the actual recording medium reproducing apparatus, thus providing satisfactory focus servo and track servo functions.
In order to obtain these effects, however, the initialization should be made track by track. Again this initializing method, therefore, requires a great deal of time. Besides, this method does not adopt a process of repeatedly irradiating the recording medium at the same position thereof with a laser beam pulse by causing several rotations of the recording medium. Therefore, the method cannot improve the phenomenon that the crystal state right after the initialization is different from that after several tens of times of over-writing with the actual recording medium reproducing apparatus.